1. Field of the Invention
This invention is primarily related to carrier type daisy wheel print element printers wherein the carrier is escaped at varying velocities during printing and the print element is rotated in a start-stop mode for character selection. More specifically, this invention relates to determining and controlling the turn on time, and on time, of a print hammer which is to impact the print element during carrier escapement.
2. Description of the Priot Art
Printers which utilize a carrier carrying a rotatable daisy wheel print element are well known. Several such printers are commercially available. These printers can be classified by either focusing on print element rotation for character selection or by focusing on carrier escapement.
Focusing on print element rotation, such printers can be further divided into a first category where the print element is continually rotated and into a second category where the print element rotation is intermittent. In printers with a continually rotating print element, printing takes place when the hammer strikes the rotating print element. In printers wherein the print element intermittently rotates, the print element is rotated from rest until a desired character is aligned for printing and then rotation is stopped. No rotation occurs during actual printing.
Focusing on carrier escapement, some printers cause the carrier to stop each time printing is to occur. In other printers the carrier is moving at the instant printing occurs, and thus printing occurs on-the-fly. In both the type where the carrier is moving when printing occurs and in the type where the carrier is stopped when printing occurs, the print element may or may not be rotating at the time of printing. In some printers where the carrier is moving at a constant velocity when printing takes place, the carrier is accelerated, decelerated, or stopped between print positions along a print line on the paper being printed. This varying motion of the carrier is to accommodate selection time. That is, escapement of the carrier between print positions must be synchronized with the time required to rotate the print element to position a desired character relative to the hammer for printing on the paper. From the above, numerous combinations of carrier escapement and character selection are available.
Representative of the closest known prior art are U.S. Pat. Nos. 3,858,509 and 4,030,591 and the above cross-referenced application.
In U.S. Pat. No. 4,030,591, a number of issued and pending patents are discussed which relates generally to printers of the type mentioned above. As pointed out in this discussion, none of the references show a printer where the carrier is moving at a plurality of different velocities when printing occurs and where the firing of the print hammer is timed dependent upon the velocity of the carrier. This specific feature, which permits increased printing speed or throughput, is found in U.S. Pat. No. 4,030,591, and is also one of the features of the instant application. However, the apparatus in U.S. Pat. No. 4,030,591 does not incorporate means for varying the impact force of the print hammer with the print element in order to achieve high print quality.
In U.S. Pat. No. 3,858,509, a display wheel printer is disclosed in which the hammer impact force can be varied between "light" and "hard." However, printing does not occur on-the-fly, and therefore, there is no need to coordinate the velocity of the carrier with actuation of the print hammer.
U.S. patent application Ser. No. 863,450 now U.S. Pat. No. 4,189,246 discloses a carrier type daisy wheel printer wherein the carrier is moving and the print element is stopped during printing. The velocity of carrier escapement is variable, and the hammer impact force is varied according to the specific character to be printed. That is, printing occurs with the carrier escaping at one of a number of constant velocities which is dependent upon the selection time of the print element. The hammer impact force for printing is varied dependent upon the character to be printed. The time of firing is dependent upon the character to be printed and the velocity of the carrier. This referenced application and U.S. Pat. No. 4,030,591 disclose operation of a daisy wheel printer in a manner particularly suitable to the hammer control of the instant application.